A variety of chromium containing aqueous solutions have heretofore been used or proposed for treating zinc, zinc alloy, cadmium, cadmium alloy and aluminum surfaces for improving the corrosion resistance properties thereof. Such treating solutions originally contained chromium in the hexavalent state and in more recent years the chromium constituent was present as a mixture of the hexavalent and trivalent forms. The reduced toxicity of trivalent chromium and the increased simplicity and efficiency in treating waste effluents containing trivalent chromium has occasioned an increased commercial use of passivate solutions in which the chromium constituent is substantially entirely in the trivalent state. Such prior trivalent chromium passivating solutions have been found to be somewhat less effective than the traditional hexavalent chromium passivating solutions in imparting good corrosion resistance to zinc, cadmium and aluminum surfaces as well as alloys thereof and there has, accordingly, been a continuing need for further improvement in trivalent chromium passivating solutions and processes.
The excellent corrosion protection provided by hexavalent chromium passivating solutions is generally associated with a light yellow iridescent passivate film which has been recognized and embodied in ASTM specifications. Conventionally, trivalent chromium passivate films are of a clear to light-blue color and are of inferior corrosion protection than the yellow hexavalent passivate film. This problem has been further aggravated by a conversion from conventional cyanide zinc and cadmium plating processes to acid and alkaline non-cyanide electroplating baths which produce metal deposits which are not as receptive to chromium passivate treatments.
Typical of prior art compositions and processes for treating metal surfaces are those disclosed in U.S. Pat. Nos. 2,393,663; 2,559,878; 3,090,710; 3,553,034; 3,755,018; 3,795,549; 3,880,772; 3,932,198; 4,126,490; 4,171,231; British Pat. Nos. 586,517 and 1,461,244; and German Pat. No. 2,526,832.
The present invention provides a treating solution and process which is effective to impart a clear light-yellow passivate film to zinc, zinc alloy, cadmium, cadmium alloy, aluminum and magnesium surfaces which provides for improved corrosion resistance approaching or comparable to that heretofore obtained employing conventional hexavalent chromium passivating solutions. The present invention is further characterized by a process which is simple to control and operate and which is of efficient and economical operation.